Electrode positive column lamp



Oct. 24, 1933 c. H. THOMAS 1,932,025

ELECTRODE POSITIVE COLUMN LAMP Filed Dec.

SOL/D ELECTEODES .DE/LLED ELECT/EODES Patented Oct. 24, 1933 UNITEDSTATES PATENT OFFICE 1,932,025 ELECTRODE POSITIVE COLUMN mm ApplicationDecember as. 1929 Serial No. 417,091

17 Claims.

This invention relates to electric devices of the gaseous conductiontype and more particularly to gas discharge devices in which thedischarge is maintained between relatively cold electrodes.

6 It is one of the objects of this invention to improve the life,maintenance and operating efliciency of electric devices of the gaseousconduction type.

\ It is another object of this invention to provide 10 a relatively coldelectrode for a gaseous conduction device capable of becomingthermionically active when subjected to positive ion bombardment duringoperation of the device.

It is another object of this invention to provide an electrode for agaseous discharge device comprised substantially of a thermionicallyactive metal which may be incandesced at least in part by positive ionbombardment to a temperature of active electron emission withoutdeleterious sputtering.

Another object of this invention is to provide an electrode for agaseous discharge device operable in said device at relatively highcurrent densities and relatively low electrode potentials.

Another object of this invention is to provide a gaseous dischargedevice which may be operated at relatively high electrode currentdensities and relatively low electrode potentials.

Another object of this invention is to provide 3 means for obtainingthermionic electron emission from an electrode in a gaseous dischargedevice without incandescing the same by electrical energy from anauxiliary circuit.

Other objects and advantages will become apparent as the invention ismore fully disclosed.

Heretofore in the art it has been customary to employ substantiallysolid coherent metal bodies as electrodes in gaseous discharge devices.These electrodes are substantially cold and the glow dis- 13 chargetherebetween is the result of the impressing of a potential upon theelectrodes in excess to the so-called break down voltage of the specificgas at the particular gas pressures employed and with the specificelectrode spacings'used.

l5 As a result of the gas discharge the electrodes are subjected to ionbombardment, and become.

incandesced thereby. Heretofore this incandescing of the electrode toelevated temperatures by ion bombardment resulted in deleterioussputtering of the electrode material. material reacted with or absorbedthe inert gas filling, thus reducing the gas pressure within the device,and changing the electrical characteristics of the device. The sputteredelectrode material 5 also deposited about the enclosing glass envelopeThe sputtered of the device, discoloring the same and lowering theefficiency thereof.

To avoid this deleterious electrode sputtering the maximum permissiblecurrent densities heretofore employed with the usual type electrodematerials approximated .66 amperes per square decimeter.

With solid type electrodes it is customarily noted that when the entireelectrode is covered with a glow discharge, the electrode potentialrequired to penetrate the electrode space charge sheath normallyincreases with increased electrode current density. It is, therefore,apparent that heretofore lower electrode current densities were moreeflicient.

It is well known in the art that the electrode space charge sheath andin particular the oathode space charge sheath may be in part neutralizedor eliminated by providing an electrode which is thermionically active,the negative stream of electrons flowing therefrom serving in efiect tolower the electrode potential required to bring a positive ion to theelectrode surface. Under such conditions higher electrode currentdensities may be employed than have been heretofore permissible withrelatively cold type electrodes.

Heretofore such an efiect' has been produced by employing a refractorymetal electrode such as tungsten, incandesced by the passage of anelectric current therethrough, from a source independent of theoperating potential applied to maintain a glow discharge in the device.Such a device has certain disadvantages from a commercial applicationstandpoint which it is one 90 of the objects of this invention toeliminate, the principal objection being the necessity of supplyingsuitable electrical heating current for the electrodes.

A second disadvantage is that such an electrode, being operated at arelatively high temperature is subject to the same sputtering eflectsheretofore obtained with solid relatively cold type electrodes, and dueto ,the relatively small size thereof, a materially shorter operatinglife 100 and efllciency than with the solid cold type electrodes isobtained.

In accordance with the objects of the present invention I have found,that these deleterious effects heretofore experienced from the use of105 solid or incandesced electrodes may be substantially eliminated anda gaseous discharge device operating at relatively low electrodepotentials over a wide range of relatively high electrode currentdensities may be obtained by employing a 9 special type of electrodecomprised substantially at least in part 0 a thermionically activemetal, such as thorium, zirconium, titanium, uranium and the like, andarranged so that the ion bombardment during the operating life thereofis confined to a relatively small area of said electrode, which area ispreferably interiorly located in said electrode.

I have found that when one face of a solid cold type electrode isrecessed a substantial distance, the glow discharge of a deviceincorporating the same tends to become concentrated within this recessedportion and that the relatively weak glow surrounding the outer surfaceof the electrode may be substantially suppressed and the entire glowconcentrated within the recessed portion without a material increase inthe required electrode potential to maintain the glow discharge, bysuperficially coating the exterior of the electrode with a refractoryinsulating material.

I have further found that by thus concentrating the glow dischargewithin a recessed portion of the electrode a material alteration in theelectrical characteristics of a discharge device incorporating the sameis obtained whereby I may apply to the electrode relatively largecurrent densities of the order of 5 and 6 times heretofore permissiblewithout materially increasing the electrode fall in potential requiredto maintain the discharge.

I have further determined that the deleterious sputtering efiectheretofore obtained due to the incandescing oi he electrode by ionbombar ment when excessive current densities are applied are materiallymitigated, and are substantially eliminated when the electrode iscomprised of metals of relatively low volatility or of rela tively highmelting points, even when electrode current densities as high as 8 toamperes per square decimeter are applied to the electrodes.

I have further found that by comprising the electrode at least in ofthermionically active material, that 3'. may apply to the electrode asumciently high currentdensity to efiect substantially an incandescenceoi the surface of that portion thereof which is subjected to ionbombardment, to a temperature at which the electrode'material emitsthermionic electron emission, which emission may thereby be utilized inimproving the operating characteristics of the device.

As a specific embodiment of the practice of my invention I will disclosethe application of the same to a gaseous discharge device of thepositive column type, as this type of a gaseous controde of the presentinvention upon the operating characteristics of a discharge deviceincorporating the same as compared to the operating characteristics of adevice incorporating the usual solid electrodes heretofore employed.

Referring to Fig. 1 the discharge device is comprised of a long tubularglass envelope 1 which is relatively small in diameter with respect toits length, having enlarged ends 2 within which are enclosed electrodes3 integral with support incinber 4 passing through press 5 to makeelectrical connection with current carrying conductors 8.

In the present illustration the narrow tubular portion 1, is shaped inthe form of a letter N and the enlarged portions 2 are bent at rightangles to the plane of the narrow tubular portion 1. Electrodes 3 arecomprised of metal, and in ac= cordance with the present invention arehollow tubular in form with one end closed, and the other open end '7substantially facing the channel of the tubular portion 1 of the device.

The electrode structure is shown in greater detail in Fig. 2 which is across sectional view of the same showing the hollow tubular featureonthe electrode 3 and the relative depth and diameter of the recessedportion 7 therein. I preferably comprise the electrode 301 .a solidcoherent mass of metal, and drill the recessed portion '1 therein in anyconvenient manner. As a specific embodiment of the present inventionelectrode 8 may be,

comprised of a highly reactive thermionically active rare refractorymetal such as thorium, zirconium, uranium and the like which metals arepreferably prepared by the process set forth in copendlng applicationSerial No. 717,949 filed June 5, 1924 by J. W. Marden et al., entitledDuctile thorium and the method of making the same, which application isassigned to the same assignee as the present invention.

In accordance with the present invention the solid thorium metal bodyfor example, prepared as by the above identified copendlng applicationis substantially shaped to the form of a hodow cylindrical body havingone end open which form may be most readily obtained by a hole in oneend of a cylindrical mass of thorium, the specific size of the electrodeand relative size and depth of the opening therein being dependent uponthe particular discharge device within it is to be incorporated, thedesired characteristics oi the electrode, the gas pressure employed, thedesired electrode voltages, and the like factors.

Prior to the drilling of the hole in the electrode I prefer to subjectthe exterior surface oi the electrode to oxidizing conditions, therebyimparting to the surface on adherent thorium oxide insulating coating,which effectively suppresses any glow discharge from the surface. Othersurface coatings of refractory materials may be applied however, but Ihave found that this method of applying the refractory coating to be themost simple.

A common size electrode which is useful in the type device illustratedin Fig. 1 is approximately .15 inches in diameter, about inches inlength, in one end of which is drilled a hole of about .075 inchesdiameter to a depth of about inch.

This electrode is then mounted in any convenient manner upon theelectrode support wire 4 and sealed into the glass envelope 1 of thedevice in the usual manner.

The device is then exhausted by mechanical exhaust means, the glassenvelope 1 being baked out for a period of time to eliminate deleteriousadsorbed and absorbed gases. Following exhaust the usual inert ormonatomic gas filling is admitted and the device sealed oil. Beforeadmitting the inert gas filling within the device the gases should firstbe thoroughly freed of deleterious atmospheric gases by well known priorart practices. The device is then subjected to a s asoning operationwherein the electrodes are subjected to positive ion bombardment atrelatively low current densities thereby effecting substantial cleanupof residual atmospheric gases within the device, the thorium electrodesacting as a "getter" for such gases. While it is expedient from amanufacturing standpoint to effect a prior purification of the inertgases, thorium electrodes will eifect the clean-up of relatively largeamounts of atmospheric gases.

An alternative electrode structure is shown in Fig. 3 wherein the hollowtubular electrode 3 is enclosed or coated superficially with anelectrical- 1y insulating coating 9, which may be of dissimilarrefractory metal oxide material than the metal of the electrode, such asfor example, hollow tubular electrode 3 may be comprised of zirconium,or titanium and coating 9 may be comprised of thorium oxide. Anotherspecific combination of electrode material that may be employed in thepractice of my invention is a thorium electrode coated superficiallywith refractory oxides of zirconium aluminum, magnesium and the like. Orfor example I may comprise the hollow electrode 3 of a highly refractorymetal such as tungsten and coat the interior surface of the hollowed outportion with thermionically active material, such as thorium or I mayincorporate the same as an alloyed or admixed constituent of the same.

These and many other variations from the specific hollow thoriumelectrode herein set forth in my specific embodiment are contemplated asa part of the present invention. 7

The specific advantages that are obtained by the use of the hollow typeelectrode, on the operating characteristics, life and maintenance of thedischarge device incorporating the same, are set forth in the graphdisclosed in Fig. 4.

Referring to the graph in Fig. 4, the test upon which these curves arebased was made upon two identical glow discharge devices in one of whichthere was a solid electrode of thorium approximately inches long by .15inches diameter, and in the other the same sized thorium electrodehollowed out or drilled a depth of one half inch with a holeapproximately ,.076 inches diameter. In each device the electrodes wereincorporated in opposite ends of a inch glass tubing a distance of 13mm. apart and a gas pressure of about 10.3 mm. neon introduced. Thecurves are identified as solid electrode and drilled electrode.

As may be noted in Fig. 4 the glow discharge device incorporating thesolid electrodes has a break down voltage of about 280 volts, and anoperating voltage of about 1'70 volts. The device incorporating thehollow electrode has a break down voltage of about 270 volts and anoperating voltage of about 155 volts.

The reason for this variation in break down voltage and operatingvoltagev between the two devices is believed to be due either to theeffect of the localizing of the glow discharge in the hollow portionupon the electrode field, or to the photo electric effects incident tothe use of the particular thermionically active electrode material, orto the eifects produced by reason of electron emission caused by thelocal overheating of the electrode surface as a result of the moreconcentrated positive ion bombardment of the cathode, for example, or itmay be due to a change in the electric field or to still other factorsat this time not apparent.

Whatever the true theory or reason may be, I have found in particularthat the usual operating or maintaining voltages of a positive columnlamp employing a hollow electrode is materially lower than that of asimilar lamp employing a solid peres, from which point a gradualincrease inpotential drop is again obtained until a maximum of about 170volts at to milliamperes is applied.

From thereon the voltage drop decreases again until at about 110milliamperes the voltage drop of potential across the electrodes isapproximately again at the minimum of 155 volts. From this point on thevoltage drop appears to stay constant within the range of the presenttest.

The explanation of this phenomena by reason of which I am enabled tooperate a glow discharge device at materially lower operating voltagesand materially higher electrode current densities than .have heretoforebeen permissible, is believed to be in part due to the fact that theelectrode space charge sheaths controlling the cathode and anode fall inpotential is substantially limited in extent, due to the concentrationor limitation, or the confinement of the glow discharge within theinterior of the electrode. The voltage required for a positive electronto penetrate the sheath therefore becomes a certainmaximum figure,dependent upon the size and depth of the recessed portion 7 of theelectrode, the gas pressure, the electrode composition and the likefactors.

In general the electrode space charge sheath of a solid electrodeentirely surrounds or encloses the electrode and it requires a certainminimum voltage for a positive ion to penetrate this sheath. Increasedelectrode current density usually increases the depth of this electrodesheath and also requires increased voltages to penetrate the same. Anyvoltage in excess of the amount necessary to penetrate the sheathappears to impart added velocity to the positive ion, which isdissipated as heat at the surface of the electrode upon impact of thepositive ion thereto and serves substantially as a means of raising thetemperature of the electrode.

As may be noted in the curve for the solid electrode in Fig. 2 withincreased potential the positive ion bombardment gradually raises thetemperature of the electrode to a point where electron emission isobtainable therefrom, with the resulting slight depression in the curveat A indicating increased efilciency.- At this current density thesputtering of the electrode is relatively high and the effectiveoperating life of the device is materially shortened. It is found,however, that the beneficial effect of the thermionic emission issubstantially lost at higher potentials as the depth of the cathodespace charge sheath increases with increased current density and thethermionic emission from the surface of the electrode is insufficient inamount to materially reduce the cathode drop in potential at this highercurrent density. The operating life of the device at these higherciu'rent densities is materially shortened.

In a glow discharge device incorporating a hollow cathode, wherein theglow discharge is confined to the interior recessed portion, the cathodefall in potential increases initially with increased current density inan identical manner as when a solid cathode is employed. At point markedA on the curve the usual increase in cathode drop in potential withincreased current density reaches a maximum, and with further increasein current density a decided drop in operating potential is obtained.This is believed due to the efi'ect of limiting the cathode space chargesheath within the confines of the recessed portion of the electrode. Asa result of this-limitation a certain maximum voltage only is requiredto penetrate this sheath.

Electrode potentials in excess to that required to penetrate the sheathare converted into heat energy at the inner surface of the electrodethrough bombardment by positive ions, coining local thermionic emissionspots, photo-electric effects, or by the use of the specificthermionically active material certain electrical effects not heretoforeobtainable are developed.

This depression or lowering of the cathode and anode drop in potentialwith increased current density continues until a minimum potential drop'is again obtained at point B. From point B to point C increased currentdensities again increase the cathode drop in potential, the specificcause thereof being not at this moment apparent. It is believed due to apolarizing or piling up action at the opening of the hollowed electrode.The particular degree of rise in the section of the curve between pointsB and C appears to" be dependent at least in part upon the specificdepth and diameter of the hollowed out portion oi the electrode, and tothe specific gas and gas pressures employed.

At materially higher current densities indicated at point C the cathodedrop in potential is again materially depressed, due it is believed tothe fact that the interior electrode surface has become incandcsced byion bombardment to a temperature where active electron or thermionicemission may be obtained.

The principal effect of thermionic emission as heretofore noted is tobreak down the electrode space charge sheath by the emission of a streamof negative electrons.

As the electrode space charge sheath is limited by reason of theconcentration of the same in the hollow electrode the cathode drop inpotential across the device in directly efiected and a decrease thereofis obtained. This decrease in cathode drop in potential continues topoint D which is at approximately 110 milliamperes and the curve thenflattens out and continues to remain so. It is believed that under theseoperating conditions the maximum neutralization of the space chargesheath by thermionic emission from the recessed portion of the electrodehas been obtained. At currents much above 110 milliamperes the electrodesputtering is so great that the life of the device is materiallyshortened.

The highest permissible current densities heretofore employed on thecommonly used solid coldtype electrodes in positive column dischargedevices is about 1.5 amperes per square decimcter of surface electrodearea, depending upon the specific electrode composition. At current den=sities much above this the sputtering of the electrode and the life andemciency or the device is materially shortened.

when the glow discharge is concentrated in a hollowed out portion of anelectrode, and the electrode comprised substantially of refractory orsubstantially non-vaporizable rare refractory metals materially highercurrent densities may be employed. With a hollow thorium electrode forexample, current densities of from 8.5) to 9.!) amperes per squaredecimeter of surface area have men employed without deleterioussputtering effects. The specific maximum current density that may beapplied will in part depend upon the electrode composition, and in partupon the depth and diameter of the recessed portion and upon theparticular gas and gas pressure within the device.

While I have specifically disclosed a hollow thorium electrode in thespecific embodiment of the present invention, similar beneficial resultsmay be obtained from employing hollow electrodes of the otherthermionically active rare refractory metals uranium, zirconium,titanium, etc. I also contemplate as hereinbelore set forth asalternative electrode materials the useof hollow refractory electrodescomprised for example or highly relractory metals such as tungsten, andtantalum, the recessed surface oi which may be coated superficially witha thermionically active material such as thorium, uranium, and the like.Such refractory metal electrodes may also have the more reactivethermionically active metals incorporated therewith as an alloyedconstituent or they may be also interiorly coated with other lowtemperature thermionically active material. The exterior of theelectrode may be coated with an electrically insulating material such asthorium oxide, aluminium oxide, magnesium oxide and the like, inaccordance with the electrode structure set forth in Fig. 3 herein.

It is also apparent that the specific electrode structure may be appliedwith similar advantage in other gaseous conduction devices, gasdischarge tubes, gas X-ray tubes and the like.

It is apparent, therefore, that there may be many variations anddepartures made of the specific embodiment herein disclosed withoutsubstantially departing from the nature of the invention as may be setforth in the following claims.

What is claimed is:

i. an electrode comprised of coherent thorium one face oi the electrodebeing recessed at least in part a substantial depth and the remainingfaces being surfaced with refractory insulating material.

2. A gas discharge device comprising an enclosing glass envelope, aninert gas filling and at least one interior electrode, said electrodebeing comprised of an open ended hollow body of thorium. 3. A gasdischarge device comprising an enclosing glass envelope, an inert gasfilling and at least one interior hollow open ended electrode, saidelectrode being comprised at least interiorly of thorium and exteriorlysurfaced with electrically insulating material.

4. A gas discharge device of the positive column type comprising anenclosing glass envelope, an inert gas filling and two open ended hollowspaced electrodes, said electrodes being comprised at least in part ofthorium.

5. In a gas discharge device the method of obtaining thermionic electronemission from relatively cold electrodes which comprises concentratingthe positive ion bombardment during operation of said device upon arelatively small surface area of said electrode to efiect incandescencethereof to the temperature of active thermionic electron emission.

d. An open-ended hollow metal electrode comprised of a thermionicallyactive metal body of the thorium group having one face thereof recesseda substantial depth.

7. An electrode comprised at least in part of a thermionically activemetal body of the thorium group having one face thereof recessed asubstantial depth and the remaining faces surfaced with a refractoryinsulating material.

8. A gas discharge device comprising an enclosing glass envelope,aninert gas filling and at least one interior hollow open-endedelectrode. said electrode being comprised at least in part of athermionically active metal of the thorium group.

9. A gas discharge device comprising an enclosing glass envelope, aninert gas filling and at least one interior hollow open-ended electrode,said electrode being comprised at least in part of a thermionicallyactive metal of the thorium group and exteriorly surfaced withelectrically insulative material.

10. A gas discharge .device of the positive column type comprising anenclosing glass envelope, an inert gas filling and two open-ended hollowspaced electrodes, said electrodes being comprised at least in part of athermionically active metal body of the thorium group.

11. A gas discharge device of the positive column type comprising anenclosing glass envelope, an inert gas filling and two open-ended hollowspaced electrodes comprised substantially of a thermionically activemetal of the thorium group, said device operating at relatively highelectrode current densities with relatively low electrode potentials.

12. A gas discharge device comprising an enclosing glass envelope, aninert gas filling and at least onehollow open-ended electrode, saidelectrode having at least a part of its interior surface of athermionically active material of the thorium group and exteriorlysurfaced with electrically insulating material.

13. An electrode for a gas discharge device comprised of a tubularmetallic body closed at one end and having at least a part of theinterior surface coated with a thermionically active material.

14. An electrode for a gas discharge device comprised of a tubularmetallic body closed at one end, a layer of a thermionically activematerial covering at least a portion of the interior surface and anelectrically insulative material on the exterior surface of said body.

15. A cathode for a gas discharge device comprised of thorium, one faceof said thorium cathodebeing recessed an appreciable depth and theremaining faces being surfaced with glow discharge suppressing sheathingmaterial.

16. A cathode for a gas discharge device comprising an open ended hollowthorium metal body exteriorly sheathed with dielectric insulatingmaterial.

17. A cathode for a gas discharge device comprising an open ended hollowthorium metal body exteriorly sheathed with material of relativelyhigher electrode drop in potential.

CHARLES HASTINGS THOMAS.

